Spark-plug.



E. W. STEVENS.

SPARK PLUG. APPLICATION FILED MAR.13, 1918.

Z @al UNITED sTATEs PATENT oEEIcE.

ELBBIDGE WEBSTER STEVENS, OF BALTIMORE, MARYLAND.

SPARK-PLUG.

To all whom t .may concern Be it known that I, ELBRIDGE WEBSTER STEVENS, a citizen of the United States, residing at Baltimore, in the State of Maryland, have invented certain new and useful Improvements in Spark-Plugs, of which the following is a specilication.

This invention relatesto spark plugs; and it comprises a spark plug having a threaded metal collar and a glass core carrying an electrode, said collar being provided with heat-radiating ns or lugs and being interiorly coated or plated with a soft, heat-conductive metal, the core extending above the collar for a substantial distance and the electrode being of metal of low thermal expansion and contraction; all as more fully herinafter set forth and as claimed.

The usual type of spark plug for gas and gasolene engines has a threaded metal collar, usually hexagonal to permit convenient grip by a wrench, within which is a cylindrical core of insulating material carrying an electrode. In use it is of course exposed to extreme conditions, one end being very hotl while the other is, comparatively, cool. Porcelain is always employed as the insulating material. The defects of this type of metal-porcelain plug are well known but no other materials have proved as satisfactory in actual use.

In the present invention I use a core of glass in lieu of one of porcelain; this core extending outward beyond the metal collar for some distance. The use of glass in this relation is not only economically advantageous, it being easier and cheaper to secure the necessary shapes and accurate dimensions in glass than in porcelain, but it has a number of other advantages. For one thing it enables an idea to be formed of what is going on in the engine cylinder, the light due to the explosions and to the heat shining l through the glass to a greater or less extent.

The use of glass in this relation however necessitates a number of correlated changes in the other arts of the plug. In a spark plug the insu ating core must of course have a good mechanical t with the collar' and preserve this t at all temperatures; and the electrode within the core must have a similar fit. As glass is a relatively fragile material Speciiication of Letters Patent.

when exposed to sudden localized strains or stresses, in securing these adjustments attention must be paid to this detail. The expansion of all the common metals being very much more than that of glass, if the core and the sleeve have a good mechanical fit at ordinary temperature the tendency at higher temperatures is to open the space between them. The only tendency toward producing strain on the glass from the collar is due to irregular expansion. As noted, the collar is usually hexagonal with a cylindrical bore. This means that the metal in the ring is thicker at some points than at v Patented Jan. 28, 1919.

Application tiled March 13, 1918. Serial No. 222,139.

others and in expanding to a high temperature expansion is irregular, which may result in deformation of the cylinder with strain on the glass at one point. In order to obviate this I take special measures to keep the upper portion of the collar relatively cool by providing it with heat-radiating iins or lugs, and I further copper plate or silver plate the interior at the point where it comes next the glass. This plating with silver or copper has two functions; one of aiding in transmission of the heat to the radiating fins and the other of providing a cushioning layer of relatively soft metal between the body of the collar, which is made of stiff and strong metal such as steel or brass, and the glass. I find that with proper contour of the metal collar, and with this heat distributing lining I am able to keep the collar cool enough to prevent destruction of the tight lit or danger to the glass. A more serious problem is afforded by the electrode passed through the glass core. Of necessity, this electrode must have a tight fit in the orifice going through the glass, and as it becomes hot if it be of any,

low that if either of these two metals is used by itself the glass may tend to expand away from it and open up a space at the bottom of the core. The space does not open near the to where the glass is cooler. Also, molyb enum and tungsten are both metals of very low electrical conductivity. There- `ordinarily give the metal a heavy coat fore, in using molybdenum and tungsten I copper or silver. Copper does very well. Used in this relation the copper has three different functions: it affords' aA cushioning element between the glass and the tungsten or molybdenum, both of which are very hard metals; it gives a heightened conductivity to the electrode as a whole; and since its expansion is relatively high it compensates for the low expansion of the tungsten or molybdenum, givlng the assemblage as a whole an expansion and contraction similar to that of the glass. In order to secure this compound expansion at the right rate, the copper coating should be relatively thick.

The thickness of copper should be so correlated to the expansion' of the 1particular metal used as to give the whole an expansion and contraction of about the same degree as that of the particular glass employed. Commercial glasses vary widely in their rates of expansion, high silica glasses usually having a less expansion than those rich in lead oxid or other basic oxids. Another metal which I may use and which is not only cheaper than tungsten or molybdenum but has cervtain advantages over either is a low expansion nickel-.iron wire. As is well known, though nickel and iron separately have both relatively high expansions, yet when alloyed together in certain particular proportions mixed metals can be obtained having much less expansion. By suitable proportioning, alloys can be obtained having an expansion about the same as that of glass. Such an alloy is-on the market under the name of platinite. Wires of this material may be usedfor the interior electrode. `-It is more advantageous however to use an alloy of a still lower expansion and coat it with copper or silver to afford a cushioning layer. The amount of copper should, as before, be so proportioned to the expansion of the core metal as to give the compound wire an expansion about like that of glass. The copper gives vbetter electrical conductivity and a cushioning effect. It is easier to maintain a tight joint between copper and glass than it is between glass and a harder metal such as the nickel steels; and it is easier to secure what is termed a drive t with copper and glass without injury to the glass than it is withl other metals.

In the accompanying drawing I have shown one type of spark plug illustrating my invention; The ligure is a vertical section partly in elevation.

Referring to the drawing, 1 is the body ,of the spark plug of steel or other usual metal havingthe screw threads 2 by means of whichl it is attached to the cylinder head of an engine and the contact 3 adjacent the central electrode 4 at `the ,bottom thereof. Thespark plug is exteriorly square or hexofl ' ner.

per or other high heat-conductive metal 7.

The central electrode 4 composed of a metal or an alloy or compond metal having substantially the, same rate of expansion as glass is surrounded by the glass block 8 preferably extending some distance above the top of the metal plug 1. This electrode is provided with the screw threads 9 at the top,

nut 10 and washer 11 for attachment 'of the wire in the well understood manner. As

has been explained the electrode may consist of a bar of nickel steel having the same rate of expansion as the glass surrounding it or it may consist of low expansion or low electrically conductive metals such as tungsten or molybdenum. In using these I ordinarily coat them with a substantial coating of copper or silver 12.

Useful and desirable spark plugs under the present invention may be ,made 'with merely a good mechanical t betweenl the several elements, that is between the interior electrode and the glass on the one hand and the glass body and the metal collar on the other hand. But where the metals in Contact with the glass are of such nature as readily give what may be termed a wetting union therewith, as is the case with silver and copper, the spark plug after assemblage of the several elements and. good mechanical fit may be placed in an annealing furnace and slowly heated to a high enough temperature to allow apartial ow of the glass and make a more positive union. This method has the advantage of removing any strains or stresses which may exist in the glass or be caused by grinding for fitting purposes. After heating, the assemblage is of course allowed to cool slowly in a well lmown man- What I claim is i 1. A spark plug comprising a metal collar, a glass insulating core andan electrode passing through said lass and of substantially the same rate o .expansion and'contraction under heat as said glass.

2. A spark plug com rising a finned interiorly plated metal co ar, a glass insulating core and an electrode .passing through said glass and of substantially the same rate of expansion and contraction under heat as said glass.

3. A spark plug comprising a metal collar,

v a glass insulatm core and an electrode passing through sai glass andof substantially the same rate of expansion and contraction under heat as said glass, said electrode carrying a layer of soft metal of high heat conductivity next the glass.

4. A spark plug comprising a metal collar, a glass insulating core and an electrode comprising nickel steel passing through said glass and of substantially the same rate 5 of expansion and contraction under heat as said glass. v

5. A spark plug comprising a metal collar, a glass insulating core and an electrode comprising copper-coated nickel steel passing through said glass and of substantially 10 the same rate of expansion and contraction under heat as said glass.

In testimony whereof, I aiix my signature hereto.

ELBRIDGE W. STEVENS. 

